This invention relates to a device for the transfer of a cryogenic medium between a fixed and a rotating conducting tube in general and more particularly to such a device having a sealing device which contains at least one corotating sealing element connected to the rotating conducting tube for sealing an annular space between a section of tube connected to the fixed conducting tube and a section of tube connected to the rotating conducting tube.
To cool a winding, particularly a superconducting winding to be deeply cooled in the rotor of an electric machine such as a turbo-generator, devices for the transfer of a cryogenic medium between the rotor and the fixed connecting lines must be provided. The cryogenic medium such as liquid or gaseous helium is carried to and from the field winding in the rotor through these transfer devices. For this purpose, the rotor of the machine contains an appropriate connecting head equipped with such a transfer device, also called a coupling. Designing this coupling is difficult especially in view of keeping thermal losses of the coolant loop for the field winding as low as possible and in view of small coolant leakage rates. The couplings must therefore have rotating seals of relatively low friction to seal the coolant, in particular the liquid helium, from the exterior on the one hand and, within the rotating system, to seal the inlet side from the outlet side on the other hand. In addition, the sealing devices required therefor must permit radial and axial rotor shaft play and function in a trouble free manner over extended periods of time, such as several years.
One transfer device of the kind mentioned at the outset is known from U.S. Pat. No. 4,056,745. This device, called a helium coupling, contains a hollow cylindrical, stationary housing with a tubular feed line projecting into its interior. The open end section of this feed tube is concentrically enclosed by an end section of a conducting tube rigidly joined to the rotating parts of the machine so that an annular gap is formed between these two tubes. The concentric position of the two pieces of tube within the housing is assured by bearings provided for this purpose. To prevent helium from escaping to the outside through the gap, an annular supporting part for a sealing ring extending in a direction parallel to the axis is provided on the face of the rotating tube. This sealing ring slides on a radically extending shaped sealing surface, of a structural part connected to the stationary housing.
Consequently, the sealing device of the known coupling represents a slide ring seal. In such a seal a corotating sealing element extending essentially in an axis-parallel direction slides on a washer-shaped sealing surface extending in a radial plane. To be able to well seal the space formed between the two sections of tube disposed concentrically to each other, the corotating sealing ring of the known machine must slide on the sealing surface under a certain pressure. Severe wear of the sealing ring cannot be avoided under these circumstances. This wear, which is particularly great when the seal is operated in the dry state, leads to a limitation of the operating life of the coupling because through it the seal characteristics deteriorate with progressing operating time, unless an automatic readjustment of the supporting part of the sealing ring is provided. Furthermore, such sealing rings generally consist of materials which, upon cooling of the coupling, shrink more than the latter's metallic components so that a shortening of the sealing rings occurs for this reason also. An automatic readjusting device, which can be installed only at relatively high expense, would be unnecessary if a whole row of slide ring seals with sealing rings extending in a direction parallel to the axis could be provided. Disposing such a row in a coupling is very costly, however.